Place 4 cups 1/2 inch-cubed watermelon (about 1 1/4 pounds; lightly smashed) and 6 sprigs mint (lightly crushed) in a pitcher. For still infused water, add 2 quarts of water. For sparkling water, add 1 quart of seltzer during prep and a second quart just before serving. Refrigerate 2 to 4 hours to allow the ingredients to infuse. Stir well and strain, discarding the solids. For serving, add fresh watermelon and mint sprigs for garnish and plenty of ice. The infused water will keep refrigerated for up to 2 days.
To lightly crush herbs, place them in the palm of your hand and lightly rub your hands together.